[LLVMdev] Promoting malloc to alloca

[Duncan Sands]

Hi Nick,

>>> The attribute you're looking for, "delete if result is unused" doesn't
>>> exist in LLVM. I've considered it in the past, but the truth is that
>>> most of the time I want to eliminate dead malloc's, they *do* have a
>>> use: the matching free. At some point I expect I'm going to teach LLVM
>>> to remove dead malloc+free / new+delete / new[]+delete[] pairings, but I
>>> haven't gotten around to it yet. Teaching it to kill an allocation with
>>> no uses at all will be dead simple too.
>>
>> LLVM already removes malloc+(perhaps compare with null)+free.
>
> What?
>
> declare noalias i8* @malloc() nounwind
> declare void @free(i8* nocapture) nounwind
> define i8 @test() {
> %A = call i8* @malloc()

^ malloc with wrong signature

> call void @free(i8* %A)
> ret i8 undef
> }
>
> doesn't optimize at all. While we're at it, we also don't get:
>
> declare noalias i8* @malloc() nounwind
> define i8 @test() {
> %A = call i8* @malloc()
> %B = load i8* %A
> ret i8 %B
> }
>
> either, though that's fixable by inserting llvm.lifetime.start right
> after the malloc.

Take a look at:

Instruction *InstCombiner::visitMalloc(Instruction &MI) {
   // If we have a malloc call which is only used in any amount of comparisons
   // to null and free calls, delete the calls and replace the comparisons with
   // true or false as appropriate.
   if (IsOnlyNullComparedAndFreed(MI)) {
...

Ciao,

Duncan.